Dockerfiles for Perl 5.6

[zakame]

I do mean 5.6.x. Repos that would quite like to use 5.6, as they still aim to support at least 5.6.2: https://github.com/Perl-Toolchain-Gang/ExtUtils-MakeMaker, https://github.com/Perl-Toolchain-Gang/ExtUtils-Install

Originally posted by @mohawk2 in #90 (comment)